ABSTRACT: Grazing
of mesozooplankton on phytoplankton, ciliates and other microplankton in the
Mississippi River plume was studied by on-deck zooplankton addition incubations
during March 2002. Diatoms, numerically predominated by the pennate diatom Pseudonitzschia
pseudodelicatissima, were the most abundant microplankton in the plume. We observed
that large cells of all types dominated the mesozooplankton diet and that phytoplankton
generally comprised the largest dietary component. Microzooplankton contributed
between 2 and 60% to the mesozooplankton diet. At the near-field station, P.
pseudodelicatissimi concentration was low and consumption of diatoms, ciliates
and dinoflagellates by mesozooplankton reflected available concentrations. In
the mid-field, P. pseudodelicatissimi attained very high concentrations (17,000
cells ml-1) but comprised only a small portion of mesozooplankton diet, which
was instead dominated by ciliates and dinoflagellates. At far-field stations
P. pseudodelicatissimi concentration was low but mesozooplankton clearance rates
were still higher on ciliates and dinoflagellates at these stations. This pattern
may have been established by changes in the composition of the mesozooplankton
grazer community, by the inability of some mesozoopllankton to efficiently ingest
the long ( > 100 mm) and large sized diatoms, or by the production of toxins
by P. pseudodelicatissimi that prevent it from been grazed by mesozooplankton.
Our findings are consistent with an earlier published conceptual model in that
1) the abundance of microzooplankton (ciliates) was high in the near- to mid
field and then decreased toward the far-field, in parallel with phytoplankton
stock; 2) mesozooplankton consumed large prey rather than the small, thereby
affecting the structure of the phytoplankton and microzooplankton community;
and 3) phytoplankton, dominated by diatoms, were the major food source for mesozooplankton
in the plume.